1. Field of the Invention
This invention relates to a light-metal piston for combustion engines, with certain points on the surface subjected to high temperatures, particularly on the piston head, where these parts are hard-eloxated in order to increase their resistance to heat.
2. Description of the Prior Art
It has already long been customary to improve the heat resistance of those surfaces of light-metal pistons which are locally subjected to high temperatures by electrolytically oxidizing them. This applies above all to the piston head, particularly when it is provided with a combustion cavity.
The fact is that in pistons for combustion engines subjected to high thermal strains there is a danger of the formation of cracks. These are mainly caused by local excessive temperatures accompanied by high mechanical strains due to gas forces. The crack formation mechanism manifests itself most distinctly at the edge of a combustion cavity in a piston head.
The edge of a combustion cavity is subjected to the following main loads:
(a) extreme temperature acting locally on the edge; PA1 (b) high gas forces; PA1 (c) intermittent changes, of high frequency, in the temperatures and gas forces.
The cracks occurring under these loads take a radial course over the edge of the cavity and occur with practically all customary aluminium piston alloys.
The combustion cavity situated in the piston head is subjected to temperatures which rise from the outer periphery of the piston head towards the edge of the cavity and which decrease from the latter towards the base of the cavity. Under these temperature conditions that zone of the piston head which undergoes the greatest temperature increase, i.e. the edge of the combustion cavity, is prevented, by the colder zones of material surrounding the edge of the cavity on all sides, from undergoing the amount of free expansion which would correspond to the thermal expansion coefficient of the material present and the temperature prevailing at that point. Considerable compressive stresses thus build up in the fully heated edge of the cavity. These compressive stresses cause plastic deformation of the light metal, of which the heat resistance is to this extent insufficient, in the zone of the edge of the cavity. The temperature peaks occurring in the said zone are frequently above 350.degree. C. As the strength of aluminium piston alloys usually suffers a considerably reduction at temperatures from 250.degree. onwards, such temperature peaks rapidly cause the elastic limit to be exceeded. The upsetting of the edge zone material, which occurs in the plastic deformation accompanying peak temperatures, causes tensile stresses to occur in that position when the cooling takes place. These tensile stresses, in their turn, are the cause of the radial cracks in the edge zone of the combustion cavity. In order to avoid such cracks these zones are subjected, inter alia, to the electrolytic oxidation process. The effect of a so-called hard eloxation coating of this kind may be explained as follows:
Compressive stresses build up in the zone of the hard eloxation layer. The hard eloxation layer itself, moreover, is still very largely resistant to extremely high temperatures, thanks to its mainly ceramic properties. To this extent the hard eloxation coating itself will even stand up to peak temperatures amounting, for example to over 350.degree. C. In addition, the coating has an important function to perform with regard to the non-oxidized aluminium material situated underneath it. For owing to the necessary equilibration of forces the compression stresses of the hard eloxation coating cause corresponding tensile stresses in the adjacent basic aluminium material.
These tensile stresses now act in opposition to the compressive stresses which are caused by the prevention of expansion and which start from the edge of the cavity and which cause the plastic deformation of the aluminium material, i.e. the said tensile stresses first of all have to be reduced before any tensile stress state can be obtained in that position. The result is that the maximum compressive stress level in the basic aluminium zone adjacent to the hard eloxation layer is reduced by the amount of the tensile stresses automatically introduced at that point in advance. For the oxide layer itself a high compressive stress is harmless, since owing to its ceramic properties such stresses cannot lead to any plastic deformation, even at the upper limit of the maximum temperatures occurring in these cases. In other words, the oxide layer itself is sufficiently resistant under all operating conditions.
Hard eloxation layers are particularly effective on the edges of combustion cavities. For the aforementioned compressive stresses undergo a particularly distinct reduction in the zone of the internal radii of the endangered edges. This likewise applies to flat piston heads, where the compressive stresses in the hard eloxation layer, however, have been reduced to the extent of the compressive stresses taking place for geometrical reasons on internal radii.